1. Technical Field
The present invention relates to a projection zoom lens for projecting an image, which is formed by a liquid crystal panel or the like, onto a screen and a projection type image display device including the projection zoom lens.
2. Related Art
There is a projection zoom lens which combines images from three liquid crystal panels by a prism and projects the combined images in an enlarged manner, for example. For such a projection zoom lens, a long back focus for arranging a color combining prism, good telecentricity for preventing the color unevenness, and a small F number for efficiently achieving the light from an illumination system are demanded. Furthermore, not only the above characteristics but also a low price is demanded in recent years. Examples of a projection zoom lens that meets such demands to some extent include a projection zoom lens which includes five lens groups and which has a variable power ratio of about 1.2 (refer to JP-A-2001-091829, JP-A-2004-325699, and JP-A-2006-065249).
However, although the projection zoom lens disclosed in JP-A-2001-091829 has an angle of view equivalent to a half angle of view of about 28° using about ten lenses, the aperture ratio is about 1:2.0. Accordingly, since the F number is large, the image is dark.
In the case of the projection zoom lens disclosed in JP-A-2004-325699, the aperture ratio is 1:1.74. Accordingly, since the F number is small for this kind of zoom lens, the image is relatively bright. Moreover, since nine lenses are used, the price is low. However, the half angle of view is only about 27°.
In the case of the projection zoom lens disclosed in JP-A-2006-065249, the aperture ratio is 1:1.55. Accordingly, since the F number is small, the image is relatively bright. Moreover, since the half angle of view is about 30°, a wide angle of view is realized. However, the number of lenses is 14 or 15, which is large. For this reason, the condition of a low price is not satisfied.
Thus, it was not easy to obtain a projection zoom lens that satisfies the three conditions, that is, a projection zoom lens which has a relatively wide angle of view, which is a bright optical system with a small F number, and which is low cost since the lens configuration is simple.
In recent years, a projection zoom lens capable of acquiring sufficient contrast even in a bright place and of obtaining a bright image by further reducing the F number according to miniaturization of an image display device, such as a liquid crystal panel, has been demanded. Moreover, in order to make a projector small or low cost, it is also becoming an important issue to reduce the cost of a projection zoom lens by miniaturizing the projection zoom lens or reducing the number of constituent lenses.